This invention relates to plate fin and tube type heat exchangers used in air conditioning, refrigeration and other applications. More particularly, the invention relates to a method of manufacturing a plate fin for use in such heat exchangers as well as the plate fin produced by the method.
Plate fin and tube heat exchangers are used in a wide variety of applications in which it is desired to exchange heat between two fluids, usually a pure liquid or a liquid undergoing a phase change to or from a gas, flowing within the heat exchanger tubes and a gas, usually air, flowing around the heat exchanger plate fins and tube exteriors. In such a heat exchanger, a plurality of thin plate fins are arranged parallel to each other between two tube sheets. Heat exchanger tubes pass through holes in the tube sheets and plate fins. There is a firm fit between the tubes and the plate fins so that the effective surface area, and thus the heat transfer area, of the heat exchanger tubes is increased by the area of the plate fins. Because of this increase in surface area, a plate fin and tube heat exchanger offers improved heat transfer performance over a plain tube type heat exchanger of the same size.
Prior art designers have devised numerous plate fin configurations. The configurations developed have attempted to improve the heat transfer performance of a given plate fin in two primary ways: (1) by maximizing, within the limits of the heat exchanger external dimensions, the plate fin surface area in contact with the gas flowing around the fins; and (2) by configuring the fin in such a way as to manage the distribution of fluid flow over the fin in order to minimize the thickness of a heat transfer inhibiting boundary layer on the external surfaces of the fin. One means of increasing the fin surface area is to corrugate the fin so that, for a given fin spacing, more fin surface area can be fit into the same volume. Corrugation also contributes to minimizing of the boundary layer.
Another means of promoting heat transfer by minimizing boundary layer thickness is to configure the fins with louvers or lances. A louver is a raised portion of the fin formed by first making a single slit into the fin and then raising the fin material on one side of the slit. A lance is a raised portion of the fin formed by first making two slits into the fin and then raising the fin material between the slits.
U.S. Pat. No. 4,860,822 (Sacks, issued Aug. 22, 1989), issued to the same inventor as the inventor of the present invention, describes a heat exchanger plate fin that incorporates more than one type of heat transfer performance enhancement. The '822 fin is corrugated in a sinusoidal pattern, with raised lance elements formed into the surface of the fin.
The holes in a heat exchanger plate fin through which the heat exchanger tubes pass are surrounded by collars formed in the fin. The function of the collars is twofold. First, they allow for a good mechanical bond between the fin and the tubes, thus enhancing heat transfer between the fin and the tubes. Second, the height of the tops of the collars from the base of the fin determines the spacing between adjacent fins and thus the number of fins per unit length of tube.
A heat exchanger incorporating plate fins such as the '822 fins exhibits excellent heat transfer performance. There are limitations, however, to the applications in which fins of the '822 type may be used. Specifically, there are lower limits on the thickness of the sheet feedstock that may be used as well as on tube diameter and corresponding wave length of the sinusoidal cross section; there are upper limits on the number, positioning and height of lance elements that may be formed in each enhanced heat transfer portion. These limitations arise because of the progressive stamping and forming operations by which fins of the '822 type are manufactured.
In manufacturing an '822 type fin, the first steps in the process are to form the fin collars and to impress the sinusoidal wave form into the fin. Then, the raised lances are formed in either one or two steps.
In the one step lance forming process, shaped punches cut slits into the sinusoidally formed fin. After cutting the slits, the punches continue their stroke into the fin material and displace the lance elements from the fin surface. This method requires very small clearances between mating tools, making spring loaded strippers necessary to push the lance regions back out of the die. Because these dies and strippers contain a comparatively large number of component parts, they are expensive to make and to maintain.
In the two step lance forming process, punches first cut slits into the sinusoidally formed fin. Then, in a subsequent step, shaped punches raise the fin material from the fin surface to form the raised lance elements. The shaped punches have controlled clearances between mating parts. The clearances obviate the need for spring assisted strippers and reduce the need for punch and die maintenance.
In manufacturing an '822 fin, drawing the fin collars and stamping in the sinusoidal wave form introduces localized stresses in the metal. Such stresses are greater when the amplitude of the sinusoidal pattern is increased and the wave length decreased. After slitting, relief of these localized stresses may occur through relative motion between the two edges of the slit. This motion can cause interference between adjacent edges and lead to edge burring when the lance elements are raised from the feedstock surface.
Raising the lance elements results in stretching and thinning of the metal at the ends of the lance elements. The stretching and thinning can result in tearing of the metal at those locations. Not only can a torn lance element end reduce heat transfer between the lance element and the main body of the fin, but also the reduced metal cross sectional area in the region of the lance element ends because of the thinning of the metal can reduce such heat transfer even if there is no tearing. And, of course, if both ends of the same lance element are torn, the element will become separated from the fin resulting in a loss of both the surface area and air flow advantages of the fin configuration.
The difficulties outlined above can and have been overcome in manufacturing plate fins of the '822 type by selecting a sheet feedstock of sufficient thickness and limiting the height that lance elements are raised from the sinusoidal surface so that the possibility of excessive thinning and tearing is minimized. In addition, the sinusoidal amplitude has been limited and the range of tubing sizes with which the fin is used has been limited to relatively larger diameters so that the sinusoidal wave length is long enough to avoid introduction of excessive residual stresses
The raised lance elements on plate fins of the '822 type are sited at regions of maximum amplitude in the sinusoidal wave form. To manufacture raised lances of that configuration, the slitting die need have cutters that extend only two different distances from the main body of the die. If lance elements were to be formed at sites other than those of maximum amplitude, the slitting die must have cutters that extend over a relatively wide range of distances, resulting in a very complex die that would be difficult to fabricate and maintain. Further, during stamping to raise lance elements on the "slope," the elements would tend to be pulled down the "slope" and thus may not properly strip from the female portion of the forming die as the die is retracted after the stamping operation, resulting in damaged or improperly formed lance elements.
Heat exchanger plate fins of the '822 type are very effective at improving heat transfer in a plate fin and tube exchanger. Fins having even more lance elements can offer ever better performance. The effort to achieve increased heat transfer performance and to produce even more compact heat exchangers, means that smaller diameter tubing and narrower plate fins can and are being used in fin and tube exchangers. What is needed is a method of manufacturing that will produce a fin that is similar in configuration and equivalent in heat transfer performance to an '822 type fin and is also adaptable to producing a similar fin having an increased number of lance elements and a shorter sinusoidal wave length using thinner sheet feedstock.